The present invention relates to: a sheet conveyance apparatus used in a post-processing apparatus which carries out post-processing for a sheet on which an image is formed by an image forming apparatus such as a copying machine, a facsimile machine, a printer and a scanner, a paper currency counting apparatus, and a mail sorting apparatus; and to an image forming apparatus having the sheet conveyance apparatus.
In the sheet conveyance apparatus, a deterioration of conveyance performance of a conveyance roller in pressure-contact with a sheet to convey the sheet due to a deterioration with time and particularly wear has been problems, and countering measures for the deterioration and wear have been studied.
In the Patent Document 1, it is suggested that a surface roughness Rz of a rubber roller is made 3 to 100 μm and an average tread interval Sm is made 15 to 200 μm to improve a durability of a conveyance force.
In the Patent Document 2, it is suggested that a rough surface roller is imposed onto the conveyance roller so as to print a rough surface pattern of the rough surface roller onto a surface of the conveyance roller in conveyance operation, thereby the deterioration of a friction resistance on the conveyance roller surface is prevented.
Patent document 1: Tokkaihei 10-181396
Patent document 2: Tokkai 2000-128377
In the sheet conveyance apparatus using the conveyance roller to convey the sheet, it is a problem that there is occurred a failure of conveyance such as slip due to the deterioration of conveyance force of the conveyance roller caused by the deterioration of the friction resistance of the conveyance roller.
Conventionally, when designing a sheet conveyance apparatus, to maintain a necessary conveyance force during a life span or a maintenance interval of the apparatus, initial values of roller characteristic such as surface roughness were determined based on experiences and accumulated data concerning deterioration with age of the conveyance force, and a material of the conveyance roller and surface treatment were determined to realize such initial values.
In such a design method, the necessary durability for the conveyance roller has been maintained by controlling the initial values of characteristic of the conveyance roller represented by the surface roughness.
However, by such design method, it has been becoming difficult to maintain necessary conveyance force for a long period of time.
Previously, in most cases, plain paper had been used for copying machines and printers, however in recent years for image forming apparatuses adaptable for POD (print on demand) market, a coated paper on which surface a coat layer is being formed, is popularly used besides the plain paper.
In the coated paper, a coating material including a pigment or a resin is applied on a surface of a pulp textile, thus when a impact is given, the coat material is easily broken away and paper powder having a diameter of 1 to 5 μm is created.
In sheet conveyance through the conveyance roller having a rubber surface layer, such paper powder is created by impact, then adheres and accumulates on the roller surface.
Since the paper powder adhered on the roller surface can be transferred from the roller surface to the sheet, an amount of the paper powder accumulates on the roller surface is determined by amounts of the paper powder transferred from the sheet to the roller surface and from the roller surface to the sheet. Therefore, when accumulation of the paper powder reaches to a certain level, these amounts become an equilibrium state, and the amount of the paper powder on the roller surface is saturated.
Even in such saturated state, if a surface not covered by the paper powder mainly including the coat material, namely a rubber surface is maintained on the roller surface, slip does not occur in conveyance of the coated paper.
In the saturated condition, the amount of paper powder adhering on the roller surface varies with environmental conditions such as a temperature, a humidity, and a kind of the sheet conveyed, however, the amount of the paper powder causing occurrence of slip varies with a micro structure of the surface of the conveyance roller. Therefore, the rougher condition of the rubber roller surface where a large number of micro convexo-concaves are provided can ensure the more area of the rubber surface where the paper powder including the coating material does not cover, even if the paper powder adheres on the roller surface. Accordingly, a degree of the friction resistance deterioration of the conveyance roller on which the conveyance force depends can be suppressed to lower level as more convexo-concaves are formed and the rougher surface is made.
The Patent Document 1 suggests to provide the micro convexo-concaves on the conveyance roller surface so as to suppress adhesion of the paper powder. Namely, in Patent Document 1, when the conveyance roller made up of rubber is formed, the surface of the roller is formed with the surface roughness RZ of 3 to 100 μm and the average tread distance of 15 to 200 μm.
In the above, the necessary conveyance force can be ensured at the beginning, however at a point of time where the diameter of the roller reduced by 400 μm (0.4 mm) due to a friction of roller surface, the convexo-concaves on the conveyance roller surface disappear and the conveyance force is deteriorated.
A wearing speed of the conveyance roller used particularly in a feeding separating mechanism which separates and feeds the sheet one by one is high. Thus in case of EPDM rubber roller generally used having a robber hardness of approximately 35 degree and a diameter of 30 mm, the conveyance force decreases before 50,000 pieces of A4 sheets are conveyed.
Since the interval of maintenance is set at around pieces in most cases, the sheet conveyance apparatus in Patent Document 1 cannot prevent slip due to insufficient conveyance force. Further in an image forming apparatus having the maintenance interval of 300,000 pieces, insufficient conveyance force is a serious problem.
Therefore, though the initial characteristic values are determined at appropriate values, it is difficult to realize the sheet conveyance apparatus without the occurrence of the conveyance failure for a long period of time.
In Patent document 2, a countermeasure based on study of the wear of the conveyance roller by plain paper is employed. Thus, in the patent document 2, there is suggested a grinding roller in contact with the roller surface to maintain the convexo-concaves even after the wear is progressed.
However, the countermeasure in Patent document 2 cannot maintain a sufficient conveyance force after passing a number of the sheets.
Also, such countermeasure for the wear of the roller is effective on some level for conveyance of the plain paper, however, it is not so effective against slip occurs when the coated paper is used.